Plan 9 is an operating system designed by the same people who created the original UNIX. Its development began in the late 1980's and it was a research project intended to address a variety of system scalability issues that the UNIX and LINUX kernels don't do particularly well, namely, distributed computing, distributed name spaces, and distributed file systems. Plan 9 is open source and its current and fourth major release was in 2002. It is
available as an install or LiveCD and it can be downloaded here. Note: This is an entry to our Alternative OS Contest.

Anyway, during the brief period when there was a window of opportunity for Plan 9, back in the mid 90s, AT&T was divesting itself, Pike et al were trying to position Plan 9/Lucifer as the ultimate embedded device OS, too soon, and the then current licensing terms for access to Plan 9 were too restrictive.

Rob did show me a really nice demo on a tiny embedded system that had a full networking stack and was snappy in some small amount of memory I don't recall now, but there was too much missing, it was too idiosyncratic, and for reasons I don't know, Lucent dropped the ball on using it for devices.

That's a pity, because it would have made a great basis for modern handheld wireles devices.

By the time the license issues were resolved, the lack of applications and hardware support made it a novelty item outside of the embedded community, and the lack of a champion made it uninteresting inside.

Now, for all intents, it's just another hobby OS, most notable for the fact that it was the last thing done in OS research at Bell Labs that is in any way related to Unix.